1. Field of the Invention
The present invention relates to a method of acquiring data from a multi-element detector in an infrared imaging apparatus, and more particularly to an improvement in a method of shortening a time required for measurement.
2. Prior Art
A multi-element detector using an MCT or an InSb is referred to as a Focal Plane Array (FPA) detector, and has a mechanism for scanning a photoelectric output from each element arranged one-dimensionally or two-dimensionally and is utilized for a detecting device of an infrared imaging apparatus. The FPA detector has the scanning mechanism of each element output by an MOSFET circuit and can therefore scan the elements in MHz order. If the scanning speed can be utilized, sampling can be carried out at a high speed.
In an infrared imaging apparatus comprising a Fourier transform infrared spectroscopic (FTIR) device for detecting a signal by means of the FPA detector, a light incident from a light source of the FTIR device becomes interferogram by an interferometer and is irradiated onto a sample. The light transmitted through the sample is photoelectrically converted by an FPA detector and thus generates an analog electric signal. The analog electric signal is converted into a digital signal by an A/D converter and is then transmitted to a computer, and is stored as digital data in a memory. The data are subjected to an arithmetic processing by a CPU and an infrared image of the sample is then displayed on the display based on image data obtained finally.
In an FTIR device of a continuous scan type, an interference signal of a laser modulated with the movement of a movable mirror included in an interferometer is used as a reference signal in order to accurately locate the position of the movable mirror, and the output of a detector is sampled synchronously with a sampling signal based on the reference signal.
In the case where the FPA detector is used as the detector of the FTIR device of a continuous scan type, however, the performance of the FPA detector capable of being scanned at a speed in MHz order cannot fully be exhibited because the frequency of the sampling signal is on the order of KHz when the sampling of each element is carried out synchronously with the sampling signal based on the said reference signal.
In order to carry out the sampling, the accuracy of which is correlated with the position of the movable mirror of the interferometer, there is only a method utilizing the reference signal. For example, even if the sampling is to be carried out by utilizing an internal signal in MHz order of the FPA detector, the correlation between the sampling signal and the position of the movable mirror of the interferometer cannot be taken with high precision. As a result, accurate measurement cannot be carried out.
If the outputs of all elements are read by a step scan operation, the above-mentioned problem can be avoided. However, an FTIR device of a step scan type is expensive. Therefore, there has been desired a high-speed data acquiring method of using a continuous scan type, which is available more inexpensively, to hold proper precision and to make the best of the performance of the FPA detector.